Environmental regulations mandate the lowering of sulfur levels in motor gasoline (mogas). For example, it is expected that regulations will require mogas sulfur levels of 30 ppm or less by 2006. In many cases, these sulfur levels will be achieved by hydrotreating naphtha produced from Fluid Catalytic Cracking (FCC cat naphtha), which is the largest contributor to sulfur in the mogas pool. Since sulfur in mogas can also lead to decreased performance of catalytic converters, a 30 ppm sulfur target is desirable even in cases where regulations would permit a higher level. As a result, techniques are required that reduce the sulfur in cat naphthas while at the same time minimizing the reduction of beneficial properties such as octane number.
Conventional fixed bed hydrotreating can reduce the sulfur level of cracked naphthas to very low levels. However, such hydrotreating also results in severe octane number loss due to extensive reduction of the olefin content as well as excessive consumption of hydrogen. Selective hydrotreating processes have recently been developed to avoid significant olefin saturation and octane number loss. Unfortunately, in such processes, the liberated H2S reacts with retained olefins forming mercaptan sulfur by reversion. Such processes can be conducted at severities which produce product within sulfur regulations. However, significant octane number loss also occurs.
One proposed approach for preserving octane during sulfur removal is to modify the olefin content of the feed using an olefin-modification catalyst followed by contact with a hydrodesulfurization (HDS) catalyst (U.S. Pat. No. 6,602,405). The olefin modification catalyst oligomerizes the olefins.
One recently developed method of HDS is SCANfining which is a process developed by Exxon Mobil Corporation. SCANfining is described in National Petroleum Refiners Association paper #AM-99-31 titled “Selective Cat Naphtha Hydrofining with Minimal Octane Loss” and U.S. Pat. Nos. 5,985,136 and 6,013,598. Typical SCANfining conditions include one and two-stage processes for hydrodesulfurizing a naphtha feedstock. The feedstock is contacted with a hydrodesulfurization catalyst comprised of about 1 to about 10 wt. % MoO3; and about 0.1 to abut 5 wt. % CoO; and a Co/Mo atomic ratio of about 0.1 to about 1.0; and a median pore diameter of about 60 Å to about 200 Å.
Even though SCANfining controls the degree of olefin saturation while achieving a high degree of HDS, there is still a need to improve the selectivity of the catalyst system to further reduce the degree of olefin saturation thereby further minimizing octane number loss.